1. Field of the Invention
This invention relates to bipolar transistors and, more particularly, to bipolar phototransistors that are monolithically coupled to optical waveguides.
2. Discussion of the Related Art
Monolithic integration of photodetectors and front-end amplifiers is an appealing technology for use in high-speed optical communications systems as well as in broadband receivers with bandwidth exceeding 40 GHz. [See, for example, K. Sano, et al., Radio Frequency Integrated Circuits (RFIC) Symposium, pp 313-316 (June 2004) and M. Bitter, et al., IEEE Photonics Technology Letters, Vol. 12, No. 1, pp 74-76 (January 2000), both of which are incorporated herein by reference.] Monolithically integrated optical receivers comprising p-i-n photodiodes (PDs) and/or avalanche photodetectors (APDs) and heterojunction bipolar transistors (HBTs) have been used for optical detection in optical transmission systems. [See, M. Bitter et al., supra.] However, to design both type of devices (optical receivers and transistors) in one process technology concessions must be made to the epitaxial structure, resulting in limited performance for each device. [See, for example, D. Ritter, et al., Proc. Intl. Topical Meeting Microwave Photonics, pp 337-340 (2002), which is also incorporated herein by reference.]
The heterojunction phototransistor (HPT) is an attractive alternative to p-i-n PDs or to APDs, since both the electronics and photodetector can be fabricated from the same set of epitaxial layers, which enables device performance to be optimized. Moreover, the HPT offers relatively large optical gain since the photocurrent is amplified by the internal gain of the transistor. [See, for example, A. Leven, et al., Electronics Letters, Vol. 40, No. 13, pp 833-834 (June 2004), which is also incorporated herein by reference.] Monolithic integration of HBTs and HPTs opens up the possibility of fabricating cost-effective, large-scale, InP-based, optoelectronic integrated circuits (OEICs). [See, for example, H. Kamitsuna, et al., IEEE J. of Selected Topics in Quantum Electronics, Vol. 10, No. 4, pp 673-678 (July/August 2004) and H. Kamitsuna, et al., IEEE Trans MTT, Vol. 49, No. 10, pp 1921-1925 (October 2001), both of which are incorporated herein by reference.] In most prior art designs the optical input is normal to the plane of the epitaxial layers; that is, the device is top illuminated. Top illumination is typically achieved by opening a window in the emitter to permit optical access to the base, but to accommodate the opening this design utilizes a larger emitter, which results in a significant high frequency performance degradation due to increased parasitic capacitance. [See, for example, A. Leven et al., supra.]. To ensure high speed performance device dimensions need to be scaled to reduce parasitic capacitance. [See, for example, M. Rodwell, et al., IEEE Trans Elec Dev., Vol. 48, No. 11, pp 2606-2624 (November 2001), which is also incorporated herein by reference.]
In addition, in some prior art top illuminated designs optical radiation is absorbed in the collector and/or emitter region as well as in the base region. Disadvantageously, however, absorption in the collector/emitter regions generates minority carriers (holes in an NPN transistor) that must diffuse/drift out of these regions to be collected, a relatively slow process that limits device speed.
Thus, a need remains in the art for a bipolar phototransistor design that does not significantly degrade high frequency performance.
In particular, a need remains for a bipolar phototransistor design that has a cutoff frequency in excess of about 180 GHz.